Dispersive spectrometers generate spectra by optically dispersing the incoming radiation into its frequency or spectral components, using a dispersive element such as a prism or grating. The spectrometer uses a narrow slit to isolate a beam of photons having a particular wavelength that is incident on and dispersed by the dispersive element and then detected by a detector. Conventional dispersive infrared spectrometers have at least a portion of their optical path and optical elements inside a cold cryogenic, vacuum-sealed (“cryo-vac”) dewar to reduce thermally-generated background noise in the measurements. This is because the detector is capable of “seeing” off the slit at other in-band wavelengths that are dispersed by the dispersive element. This ability to see off the slit would contribute to unacceptably high background noise on the detector and compromise performance of the spectrometer if a warm or highly emissive object were in the optical path of the detector. Therefore, the optical components on the detector side of the slit (including the detector itself) are typically placed with the cryo-vac dewar to keep the components cold and reduce thermal noise.